A crucial role of the PD-1 axis in survival and clonal selection of stem-like CD8+ T cells

Introduction

For cancer cells to successfully establish and grow, they must overcome immune surveillance and immune mediated destruction. One crucial arm of the immune system, cytotoxic T, or CD8+ T cells, are critical players in tumor control. In tumors, naïve CD8+ T cells are primed in the tumor draining lymph node (TDLN) through recognition via unique T cell receptor (TCR) expressed on their surface by tumor antigens presented to them by antigen presenting cells. Following activation, differentiated CD8+ T cells then mediate direct cancer cell killing through release of granzymes and perforin.

However, inhibitory molecules that pump the brakes on TCR signaling restrain T cell differentiation and anti-tumor function. One such signaling axis, PD-1, and its ligands PD-L1 and PD-L2, have become a target for releasing the brakes on T cells and enhancing their capacity to kill tumor cells. While immune checkpoint blockade (ICB) has shown immense success in recent years for various cancers, only a subset of patients responds and eventually relapse, highlighting a critical gap in our understanding of how PD-1 fine tunes CD8+ T cell responses.

Stem-like progenitor CD8+ T cells

While this model of CD8+ T cell priming and tumor infiltration explains some aspects of anti-tumor immunity, this model does not fully capture the diversity of T cell states within the tumor. Recent evidence has identified the presence of a subset of CD8+ T cells known as stem-like progenitor cells, which have been identified by expression of the transcription factor TCF-1, and expression of PD-1, does not fit this model, as this T cell subset resides primarily in the TDLN, have recognized antigen, and proliferated, yet do not exhibit the canonical cytotoxic function of differentiated CD8+ T cells. Instead, these stem-like CD8+ T cells serve as a pool of precursor T cells in the TDLN that can differentiate into effector T cells during ICB.

Goals of this study

This raises important questions regarding maintenance of T cell states during chronic antigen exposure. In this study published in Nature, the researchers set out to address these questions:

• What maintains these stem-like states of activated T cells in TDLNs?
• Is T cell fate imprinted early or is it being constantly shaped in the TDLNs?

The researchers used a model of subcutaneously implanted lung cancer cells expressing ovalbumin, followed by adoptive transfer of OT-1 T cells into XCR1 reporter mice. This specific model allowed the researchers to be able to mark dendritic cells through use of XCR1 and allowed them to follow tumor specific CD8+ T cells using the ovalbumin-OT-1 model. Through multiplex immunofluorescence imaging of the TDLN and flow cytometry, they found that these stem-like CD8+ T cells, marked by TCF-1, PD-1, and SLAMF6, another marker for stem-like state, were near dendritic cells, an antigen presenting cell, and restricted to the TDLN. These T cells, which were clustered with dendritic cells, had already undergone activation and proliferation, and continued to be exposed to antigen even at later stages.

Furthermore, the authors utilized H-2Kᵇ–SIINFEKL tetramer staining to identify OVA-specific CD8⁺ T cells and generate a measure of TCR binding avidity. Interestingly, stem-like T cells with the highest TCR binding affinity also expressed the highest levels of PD-1, suggesting that PD-1 signaling may play a role in the evolution or maintenance of higher-affinity T cell clones that are better equipped to respond to tumor. Dendritic cells expressed the PD-1 ligand PD-L1, and depletion of dendritic cells reduced the stem-like CD8⁺ T cell pool. To more directly test whether blocking PD-1 depleted this population, mice were treated with PD-1 checkpoint blockade. Disruption of PD-1 signaling reduced the pool of high-affinity stem-like CD8⁺ T cell clones capable of mediating a strong anti-tumor immune response.

Overall, this study has important clinical implications for immune checkpoint blockade (ICB) therapy. Disrupting the PD-1 pathway that maintains high-affinity stem-like CD8⁺ T cells may lead to loss of this progenitor pool and potentially compromise therapeutic efficacy if lower-affinity clones instead gain access to the tumor. These findings also raise the possibility that current immunotherapies may provide short-term benefits at the cost of diminished efficacy over time due to depletion of high-affinity progenitors.

To me, this paper helps clarify the biological role of PD-1 signaling beyond its use as a marker of exhausted or dysfunctional T cells. The study convincingly demonstrates in preclinical tumor models that PD-1 signaling is protective for the maintenance of stem-like CD8⁺ T cells, and that disrupting this axis through ICB leads to depletion of higher-affinity TCR clones. However, it remains unclear whether these findings extend to other tumor types, or how these higher-affinity T cell clones overcome the immunosuppressive tumor microenvironment once they infiltrate the tumor. Moreover, this work raises the question of whether altering the timing of immunotherapy as a more cyclical treatment strategy to potentially allow time for these stem-like T cell pools to recover before another round of ICB could improve clinical outcomes.

Article title: “Inhibitory PD-1 axis maintains high-avidity stem-like CD8+ T cells.”

Article Reference: Hor, J.L., Schrom, E.C., Wong-Rolle, A. et al. Inhibitory PD-1 axis maintains high-avidity stem-like CD8+ T cells. Nature (2025). https://doi.org/10.1038/s41586-025-09440-x

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